The boys have a new fellow student in their class, the mentally and physically challenged Timmy, who is only capable of making various noises, saying his own name, and "LIVIN-A-LIE". Mr. Garrison and Principal Victoria ignore that he has a disability, even though his handicaps are obvious. Mr. Mackey suggests that Timmy may suffer from ADD (Attention Deficit Disorder). They send him to a doctor who just diagnoses him with that "disease" in a very odd fashion. Timmy is freed from all homework. All the other kids in the class claim they have ADD, because they also want to be rid of homework, and they are all promptly diagnosed as well, and are prescribed Ritalin.
Without the burden of homework, Timmy finds a new pastime as he is discovered by a band named The Lords of The Underworld (see "Cat Orgy"), which takes him on as its new lead singer. The band becomes instantly successful due to Timmy's antics. However, many people are upset as they think that Timmy is being ridiculed. Phil Collins in particular is displeased with the new band, which has been booked to open for him at the Lalapalalapaza festival. Shortly after, Timmy and the Lords of the Underworld take Collins's place as the headliners of the festival.
The other boys have actually started to take their Ritalin medication, making them very calm and rather boring. Cartman develops a side effect that causes him to see pink Christina Aguilera monsters. The adults are uncomfortable among them, but accept their new kind and obedient children when they also start taking Ritalin. Chef and the pharmacists are the only people left who are not under the drug's influence.
Meanwhile, Phil Collins tries to break up Timmy and the Lords of the Underworld. First by appealing to Timmy's parents, Richard and Helen Burch (who are also handicapped in exactly the same ways as Timmy), and later by telling the guitarist, Skyler, that Timmy is stealing his fame and is only holding Skyler back. Skyler leaves the band, which is subsequently cancelled from the festival. Collins regains his headlining spot, and Skyler's solo project Reach for the Skyler is booked to open for him.
In the meantime, Chef tries to convince the parents that there are other methods to fight ADD than medication, but as the parents are all taking Ritalin too, he doesn't get any help and is forced to confront the pharmacists alone. After the boys come in and tell Chef that they want to go to the festival to see Phil Collins perform, Chef confronts the pharmacists and tells them that they are responsible for the children liking Collins. Horrified that they are responsible for this, they make a plan to distribute an antidote called "Ritalout" by mixing it into free drinks at the Lalapalalapaza festival. They get the drinks from a lemonade stand run by Mr. Derp (a minor character who appeared last in The Succubus).
The plan works perfectly; Collins is booed from the stage, and the crowd starts chanting for Timmy. The band reunites with Skyler and they play their show. Timmy even learns the words to introduce as "Timmy and the Lords Of The Underworld"—much to Skyler's pleasure, as everyone cheers happily as the music rolls through the credits.